This invention relates to cyclone combustor apparatus, and more particularly, to air cooled apparatus for optimum combustion of coal, and the reduction, over conventional combustors, of the pollutants resulting from coal combustion. The present apparatus may be used to practice the processes described in a copending application entitled, "Method of Optimizing Combustion and the Capture of Pollutants during coal Combustion in a Cyclone Combustor", assigned to the assignee of the present application.
A cyclone coal combustor is, in general, a horizontal cylindrical device into which pulverized coal is injected with primary air, the air-coal mixture then to be centrifuged with secondary air toward the cylindrical wall of the cyclone. When coal particles burn while in suspension or on the wall of the cyclone in hot oxidizing gas temperature (at average temperatures around 3000.degree. F.), the ash particles in the coal melt. Those in suspension are thrown to the wall. This liquified ash, called slag, rapidly coats the wall, and is continuously drained by the action of gravity toward the bottom and downstream end of the cyclone. There, in conventional practice, it is removed through a port called a slag tap.
In the above-mentioned copending application, it is disclosed that the separate injection of coal particles and limestone particles (both having an appropriate size range), and the injection of the air (at the proper temperature and swirling velocity, and in an amount to achieve a desired air/coal fuel ratio) will result in combustion inside the combustor under conditions in which: (1) the limestone reacts with and removes most of the sulfur gas compounds released by the coal; (2) almost all the slag released by the coal is retained on the wall for subsequent removal; and (3) the emission from the coal of the fuel-bound nitrogen compounds is controlled in a manner that will allow their subsequent conversion to nitrogen in the furnace to which the cyclone is attached.
While commercial horizontal cyclone combustors have heretofore been used to remove 70 to 85 percent of the coal ash, the processes disclosed in the above-mentioned copending application can achieve even higher ash removal, along with efficient nitrogen and sulfur pollutant control.
It is therefore a general object of the present invention to provide a combustor capable of practicing the processes disclosed in the above-mentioned copending application.
It is another object of this invention to provide an air-cooled cyclone coal combustor which is capable of (1) optimizing the combustion of the volatile and carbon compounds in coal; (2) maximizing the capture of gaseous compounds of sulfur, and (3) retaining solid and liquid particles from the gas stream before they are exhausted from the combustor.
As was explained in the above-mentioned copending application, an important feature needed to limit sulfur gas evolution from the slag and to prevent corrosive attack of the combustor walls is a thin, completely liquid slag layer, which flows relatively rapidly along the combustor's inner walls. With air cooling, one can maintain such a slag layer for a wide range of combustor operating conditions and different coal types. With water cooling, on the other hand, the slag layer tends to be thick, and frozen over most of its thickness due to contact with the water-cooled metal wall.
It has heretofore been proposed to (1) use in a cyclone coal combustor a non-sacrificial ceramic inner liner, to be coated during operation with a thin liquid slag layer, and (2) to use injection at the closed end of the cyclone of the coal, limestone and air streams. These features were first described in relation to a test program involving a 1 million BTU/hr coal capacity air cooled laboratory cyclone combustor. Later researchers found, however, that for a number of reasons, the design of the 1 million BTU/hr laboratory combustor (which had a 1 foot internal diameter and 2 feet internal length) could not simply be scaled up to commercial size combustors.
Specifically, the air cooled liner in the 1 million BTU/hour laboratory cyclone combustor used an externally grooved metal cylinder, which enclosed a liner made of ceramic cement. The grooves, which were parallel to the horizontal axis of the cyclone, carried the liner cooling air. While such a liner arrangement is satisfactory at 1 million BTU/hr, at larger sizes the different expansion coefficients of metal and ceramic cause the metal shell to separate from the ceramic in both the tangential and circumferential directions. This results in uneven liner cooling and liquid slag flow out of the combustion chamber at the upstream and downstream bottom edges of the liner, with attendant damage to the apparatus.
A second feature of the 1 million BTU/hour laboratory cyclone combustor which limits scale-up of its design is the method used for injection of the secondary combustion air. The secondary combustion air was injected at the closed end of the cyclone in a circle lying inside the coal injection circle. A series of helically curved swirl vanes directed the secondary air, but at about 70 million BTU/hr the air velocity required with such an arrangement approaches the speed of sound, resulting in an unacceptable pressure loss.
Another complicating factor in the design of a coal combustor is the fact that the liquid slag temperature required for optimum operation changes with coal type. With water cooling, the only method by which the cyclone can be adjusted to different coals and different operating conditions is changing of the slag layer thickness. Air cooling, however, provides more flexibility. An example of the flexibility of air cooling is given for the 1 million BTU/hour cyclone: A factor of two change in the air cooling mass flow rate changes the slag-ceramic interface temperature by 400.degree. F. For a similar design with water cooling, a 50.degree. F. temperature change requires a factor of three change in the water flow rate. While water cooling, which has been used in prior large commercial cyclones, can eliminate the liner expansion problem, it greatly limits the flexibility of the cyclone to operate in the optimum combustion and pollutant control mode with a wide range of coals.
Still another advantage of the air cooling in coal combustors is that the heat lost from the combution gases in the cyclone can be regenerated through the cooling air back into the combustor, with the cooling air being used as part of the secondary air supply.
The principles of present invention may be applied to combustors in size ranges from less than about one million BTU/hr to over 100 million BTU/hr, although the detailed description set forth below relates to a combustor rated at approximately 50 million BTU/hr at 15 percent excess air operation and 100 million BTU/hr at 70 percent of the stoichiometric air/fuel ratio.